Published in Astronomy & Astrophysics Review, 7, 1-54, 1996.
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CYGNUS A

C.L. Carilli 1,2 and P.D. Barthel 3


1 Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands
2 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
3 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands


Summary. Cygnus A was the first hyper-active galaxy discovered, and it remains by far the closest of the ultra-luminous radio galaxies. As such, Cygnus A has played a fundamental role in the study of virtually all aspects of extreme activity in galaxies. We present a review of jet theory for powering the double-lobed radio emitting structures in powerful radio galaxies, followed by a review of observations of Cygnus A in the radio, optical, and X-ray relevant to testing various aspects of jet theory. Issues addressed include: jet structure from pc- to kpc-scales, jet stability, confinement, composition, and velocity, the double shock structure for the jet terminus and the origin of multiple radio hotspots, the nature of the filamentary structure in the radio lobes, and the hydrodynamic evolution of the radio lobes within a dense cluster atmosphere, including an analysis of pressure balance between the various gaseous components. Also discussed are relativistic particle acceleration and loss mechanisms in Cygnus A, as well as magnetic field strengths and geometries both within the radio source, and in the intracluster medium. We subsequently review the classification, cluster membership, and the emission components of the Cygnus A galaxy. The origin of the activity is discussed. Concentrating on the nuclear regions of the galaxy, we review the evidence for an obscured QSO, also given the constraints on the orientation of the radio source axis with respect to the sky plane. We present an overview of models of central engines in AGN and observations of Cygnus A which may be relevant to testing such models. We conclude with a brief section concerning the question of whether Cygnus A is representative of powerful high redshift radio galaxies.

Key words: Galaxies: active - galaxies: individual: Cygnus A - radio continuum: galaxies


Table of Contents

INTRODUCTION

JET THEORY
The Standard Model
Simulations

THE JETS IN CYGNUS A FROM PC- TO KPC-SCALES
Images: Bends and Knots
Opening Angle, Stability, and Confinement
Velocity and Composition

THE HOTSPOTS
The Dentist's Drill

THE RADIO LOBES
The Bridge and Plumes
The Filaments

RADIO CONTINUUM SPECTROSCOPY
Synchrotron Spectral Ageing
The Hotspot Spectra
Lobe Spectra and the Age of the Radio Source
An Alternative: The Universal Spectrum

MAGNETIC FIELDS
Minimum Energy
Fractional Polarization and Projected Field Morphology
The Faraday Screen: Magnetic Fields in Cluster Gas

THE X-RAY CLUSTER
Large Scale Gas Distribution
The Second Dynamical Component
Pressure Balance
X-Ray Hotspots

THE CYGNUS A GALAXY
Classification and Cluster Membership
Optical Emission Components
Infrared, Submm, and MM Emission
A Merger Origin for the Cygnus A Galaxy?
A Cooling Flow Origin for the Cygnus A Activity?

THE NUCLEAR REGIONS OF THE CYGNUS A GALAXY

AN OBSCURED QSO IN CYGNUS A?

THE SYMMETRIC TWIN RELATIVISTIC JET MODEL APPLIED TO CYGNUS A

THE CENTRAL ENGINE
Some Theories
Some Observations: Variability, Small-Scale Structure

IS CYGNUS A REPRESENTATIVE OF z ~ 1 GALAXIES?

REFERENCES

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